1. Field of the Invention
This invention relates to a photodiode as a light receiving device and photodiode module combining a photodiode and a light emitting device for bilateral or unilateral optical communication of information between a base station and a plurality of subscribers by transmitting two light signals having different wavelengths xcex1 and xcex2 in a unilateral direction or bilateral directions passing through an optical fiber.
This application claims the priority with respect to Japanese Patent Application No.256107/1997 filed on Sep. 3, 1997 which is incorporated herein by reference.
2. Description of the Related Art
Explanation of Optical Bilateral Communication
Recently semiconductor laser diodes (LD) and semiconductor photodiodes (PD) have enhanced their properties and the transmission loss of optical fibers has been lowered. Therefore, the transmission of various sorts of information by light have been feasible. Transmission by using light is referred to as xe2x80x9coptical communicationxe2x80x9d. Telephones, facsimiles, television image signals and so on are various known media for transmitting information. Various attempts to utilize light with long wavelengths, for example, a wavelength of 1.3 xcexcm and wavelengths of 1.5 to 1.6 xcexcm, are succeeding in the optical communication field. Nowadays, a new system for sending and receiving signals is being developed, in which light signals can be sent in bilateral directions simultaneously through a single optical fiber. This system is called xe2x80x9cbilateral communicationxe2x80x9d, because signals can be transmitted in bilateral directions. The most noticeable advantage is that the bilateral communication requires only one optical fiber.
FIG. 1 explains the principle of the bilateral multicommunication using two light signals of different wavelengths. An optical fiber connects one base station to one of many subscribers. In FIG. 1, only one subscriber is shown for simplicity, but actually, there are many branch points, and an optical fiber from the base station is branched into a plurality of optical fibers that are connected to respective subscriber devices.
In the base station, the signal from a TV, a phone or the like is amplified and processed to be a digital or an analog signal, and such an amplified signal drives a laser diode (LD1) of xcex1. The light signal having a wavelength xcex1 enters an optical fiber 1, and then to an intermediate optical fiber 3 via a wavelength division multiplexer (WDM) 2. The signal introduced in the optical fiber 3 is transmitted to an optical fiber 5 via a wavelength division multiplexer (WDM) 4 set up on the subscriber side, and is received by a photodiode (PD2), whereby the signal is changed to an electric signal (P3). The electric signal (P3) is amplified and processed by a device on the subscriber side, and is regenerated as the voice of a phone or television images. The signal going from the base station to the subscriber side is called a xe2x80x9cdownward signalxe2x80x9d, and the direction of the flow is called a xe2x80x9cdownward directionxe2x80x9d.
On the other hand, on the side of a subscriber, a laser diode LD2 converts a signal (P4) from a phone or a facsimile into a light signal having a wavelength of xcex2. The xcex2 light enters an optical fiber 6, is introduced into the intermediate optical fiber 3 by the wavelength division multiplexer (WDM) 4, and enters photodiode PD1 passing through the wavelength division multiplexer 2 of the base station. A device equipped in the base station converts the xcex2 light signal to an electric signal using photodiode PD1. This electric signal is appropriately converted and/or processed. The direction from the subscriber side to the base station is called an xe2x80x9cupward directionxe2x80x9d.
Explanation of Wavelength Division Multiplex
Both the base station and the subscriber side must be able to distinguish and separate two sorts of light having different wavelengths for the implementation of optical bilateral communication using a single optical fiber. The wavelength division multiplexers 2 and 4 shown in FIG. 1 provide that capability. Such a wavelength division multiplexer combines two sorts of light having different wavelengths of xcex1 and xcex2 and couples the combined light into only one optical fiber or selects one sort of light from light having different wavelengths, and couples the light of one wavelength to only one optical fiber. Therefore, the wavelength division multiplexer plays an extremely important role in carrying out multiwavelength bilateral communication.
There are several kinds of wavelength division multiplexers, which have been proposed. These wavelength division multiplexers will be explained by referring to FIG. 2 to FIG. 4. A wavelength division multiplexer shown by FIG. 2 is made from optical fibers or an optical waveguide. Two optical paths 8 and 9 are close to each other at a part 10 of the paths, and the exchange of optical energy is carried out here. Various kinds of couplings can be realized by changing gap D and length L of the neighboring part 10.
When the light having a wavelength xcex1 enters the optical path 8 and the light with a wavelength xcex2 enters the optical path 9, both the xcex1 wavelength light and the xcex2 wavelength light exit via an optical path 11, and no light enters an optical path 12. Hence, the xcex1 light from a port P1 and the xcex2 light from a port P2 appear in a port P3. No light appears in a port P4. The xcex1 light never enters the neighboring optical fiber, but all of the xcex2 light enters the neighboring optical fiber because the xcex2 satisfies the condition of phase. Since such a wavelength division multiplexer is made from optical fibers or an optical waveguide, there is an advantage of little polarization dependency.
The route of light passing through an optical fiber or a waveguide has reversibility. FIG. 3 shows a practical device for the wavelength division multiplexer shown in FIG. 2 in the bilateral communication. The xcex1 light from P1 enters the optical fiber 8, and exits via P3. The xcex2 light entering P3 exits via P2. Here, this WDM can be used as the WDMs 2 and 4 shown in FIG. 1.
FIG. 4 shows a wavelength division multiplexer provided with a multi-layer mirror. This multi-layer mirror is made of two glass blocks 13 and 14 shaped like isosceles triangle columns and a dielectric multi-layer 15 formed on the slanting surfaces of the glass blocks 13 and 14. All of the xcex1 light passes through but the all of the xcex2 light is reflected on the multi-layer mirror by adjusting and combining the refractive index and the thickness of the dielectric multi-layer. Here, there exists the dependency of polarization because the incoming light is reflected at an angle of 45xc2x0. This wavelength division multiplexer is utilized as the wavelength division multiplexers 2 and 4 shown in FIG. 1. Such a kind of wavelength division multiplexer is called a xe2x80x9cwavelength dividing or combining devicexe2x80x9d or called a xe2x80x9cWDMxe2x80x9d. The optical fiber type WDM and the glass block type WDM mentioned above have already been on the market.
The light sending and receiving module on the subscriber side will be explained by referring to FIG. 5. In FIG. 5, an end of an optical fiber 16 leading from the base station to each subscriber is connected to an indoor optical fiber 18 by an optical connector 17. An indoor ONU module equipped on the subscriber side is provided with an optical fiber WDM (Wavelength Division Multiplexer) 21. The WDM has another optical fiber 19 which is connected to the fiber 18 at a coupling portion 20 via evanescent waves. The optical fibers 18 and 19 are wavelength-selectively coupled with modules in the WDM 21. The optical fiber 18 is coupled with an LD module 25 by an optical connector 22. The optical fiber 19 is coupled with a PD module 27 via an optical connector 23.
The LD 25 and the optical fiber 24 are of the upward-direction type. The 1.3 xcexcm-band light is transmitted from the subscriber side to the base station. The optical fiber 26 and the PD module 27 are of the downward-direction type. For example, the 1.55 xcexcm-band light signal from the base station is converted into an electric signal by the PD module. The LD 25, which is a transmitting device, includes circuits capable of amplifying and modifying the signals of telephones and facsimiles and a semiconductor laser capable of converting electric signals to light signals. The PD module 27, which is a light receiving device, includes a photodiode capable of converting light signals such like TV signals from the base station, telephone and so on to electric signals, an amplifier, a demodulator and so on.
The wavelength division multiplexer 21 can separate the 1.55 xcexcm-band light from the 1.3 xcexcm-band light. In this example, the 1.3 xcexcm-band light is used as an upward-direction signal, and the 1.55 xcexcm-band light is used as a downward-direction signal. In this case, light having two different wavelengths must effectively be separated by the wavelength division multiplexer.
This invention relates to an improvement for both a photodiode and a photodiode module for use in bilateral communication of using light signals having two different wavelengths.
Explanation of Prior Photodiode Module
A known semiconductor photodiode module will be explained by referring to FIG. 6. In FIG. 6, a photodiode chip 41 is fitted on an upper surface of a header 42 by die-bonding. The bottom surface of the header 42 is provided with lead pins 43. The upper surface of the header 42 is covered with a cap 44. A window 45 for penetrating light is formed at the center part of the cap 44. A cylindrical holder 46 is fixed to the header 42 outside of the cap 44. The holder 46 is necessary for supporting a lens 47.
A conical housing 48 is fixed on the lens holder 46. An edge of an optical fiber 50 is settled by a ferrule 49 which is sustained by the housing 48. The tips 51 of the ferrule 49 and the optical fiber 50 are slantingly polished.
The positions of the holder 46, the housing 48 and the ferrule 49 are determined for the alignment of the photodiode chip 41, for conducting light into the optical fiber 50 and investigating the output of the photodiode chip 41. The wavelength feasible to be received is decided by semiconductor layers of the photodiode. In the case of visible rays, the photodiode made from silicon (Si) can be used. However, since this invention is intended for ONU modules using near infrared-rays, Si-photodiodes are unsuitable. A compound semiconductor photodiode having an InP substrate is required for sensing infrared-rays.
Explanation of Prior Semiconductor Photodiode Chip
A conventional photodiode, which is used for the light having long wavelengths and has an InP substrate, will be explained. FIG. 7 is a sectional view of a known photodiode chip for light communication.
An n-type InP buffer layer 53, an n-type InGaAs light receiving layer (light absorbing layer) 54 and an n-type InP window layer 55 are epitaxially grown in this order on an n-type InP substrate 52. A p-type region 56 is formed by diffusing zinc (Zn) into the upper central part of the chip. The Zn-diffusion reaches to the middle depth of the n-type InGaAs light receiving layer. Hence, the Zn-diffiusion area becomes a p-type InP window layer and a p-type InGaAs light receiving layer. There occurs a pn-junction inside of the InGaAs light receiving layer.
A ring p-electrode 57 is fitted on the Zn-diffusion region 56. Light enters the inner part enclosed by the ring p-electrode 57. The light incident part is covered with an antireflection film 58. The outside of the p-electrode 57 is protected with a passivation film 59. An n-electrode 60 is formed on the rear surface of the substrate 52. Since the electrode is surely formed on the whole rear surface of the substrate, no light penetrates. This is one of the characteristics of conventional photodiodes.
In operation, the pn-junction is reverse-biased by supplying voltage between the p-electrode 57 and the n-electrode 60. Incident light emitted from the end of the optical fiber reaches the n-type InGaAs light receiving layer 54 passing through the anti-reflection film 58 at the center part, the p-type InP window layer, the p-type InGaAs light receiving layer and the pn-junction. The generation of electron-hole pairs is derived from the absorption of light into the light receiving layer. Electrons transfer toward the n-electrode and the holes move toward the p-electrode because of the reverse-bias of the pn-junction, and whereby current flows between the n-electrode and the p-electrode. The power of incident light is possible to be detected because the amount of absorbed photons corresponds to the current therebetween.
The InGaAs light absorbing layer can absorb the light having a wavelength of 1.3 xcexcm and a wavelength of from 1.5 xcexcm to 1.6 xcexcm. The InP window layer never absorbs the light. A semiconductor has an inherent feature that it is permeable to light having a smaller energy than the bandgap, because the light cannot lift electrons of the valence band up to the conduction band. Hence, the semiconductor is a transparent to the light having a longer wavelength than the wavelength corresponding to the bandgap.
On the contrary, if the semiconductor has a sufficient thickness, the semiconductor can absorb all of the light having larger energy than the bandgap. Such light can lift electrons of the valence band up to the conduction band. When the bandgap of the semiconductor of the window layer is denoted by Egw, and the bandgap of the light absorbing layer (light receiving layer) is denoted by Egz, the light having an energy more than Egz and less than Egw, that is, Egz less than hxcexd less than Egw, can pass the window layer and is sensed by the light receiving layer. Here, h is Planck""s constant and xcexd is an oscillation frequency (frequency). This photodiode is detectable for the light having an energy that is larger than the bandgap Egz of the absorbing layer and smaller than the bandgap Egw of the window layer.
Furthermore, in the InGaAs light receiving layer, the rate between In and Ga is predetermined from the condition of lattice matching between the InP substrate and the InGaAs light receiving layer. When InGaAs is written as In1-xGaxAs, the mixed crystal rate x is determined to be one value. Hence, the bandgap of the InGaAs layer matching with the InP is determined from the value.
FIG. 8 shows the sensitivity property of the InGaAs photodiode shown by FIG. 7. The abscissa is the wavelength of light (xcexcm) and the ordinate is the sensitivity (A/W). The sensitivity is low in the range less than 0.9 xcexcm (P-region) and rises steeply at a wavelength of 0.95 xcexcm. The sensitivity is increased monotonously in the range of from 1.0 xcexcm to 1.5 xcexcm (Q-region). The sensitivity goes down sharply from a wavelength of 1.7 xcexcm (R-region) and the sensitivity falls down to zero at a wavelength of 1.75 xcexcm.
As is well known, the equation h xcexd=hc/xcex=E is established between the wavelength xcex and the energy E. Here, h is Planck""s constant, xcexd is an oscillation frequency of light and c is the velocity of light. The bandgap Egw of the window layer (InP) determines the lowest limit of wavelength of sensitivity (0.95 xcexcm). Since the light having energy higher than Egw is entirely absorbed by the window layer, the light never reaches the light receiving layer (absorbing layer).
The bandgap Egz of the absorbing layer (InGaAs) determines the upper limit of wavelength of sensitivity (1.67 xcexcm). Since the light having energy less than Egz passes through the absorbing layer, the detector never senses the light.
The wavelength of P-region of FIG. 8 at which the sensitivity rises depends on the bandgap of the window layer, and the wavelength of R-region at which the sensitivity goes down depends on the bandgap of the absorbing layer.
Since this photodiode has such a wide range of sensitivity, it has enough to sense both the 1.3 xcexcm light and the 1.55 xcexcm light. Hence, the same photodiode can be used for detecting both the 1.3 xcexcm light and the 1.55 xcexcm light.
Further, the energy of a photon is h xcexd=hc/xcex. Theoretically, one photon makes an electron-hole pair and there occurs a current of 2q (q: elementary quantum of electronic charge), which would be realizable if the conversion efficiency were 100%. In other words, when the efficiency of the photodiode is 100%, the sensitivity is given by 2q xcex/hc (A/W). This fact is related to the sensitivity increasing monotonously in Q-region ranging from 1.0 xcexcm to 1.55 xcexcm in the wavelength of xcex, shown in FIG. 8. A photodiode with a high sensitivity surely forms such a sensitivity curve.
In the system of transmitting signals of light communication by using the 1.3 xcexcm light and the 1.55 xcexcm light, it would be extremely convenient to use a photodiode capable of sensing both the light having different wavelengths of 1.3 xcexcm and 1.55 xcexcm. However, there is an impediment here.
The other impediment exists in the wavelength division multiplexer.
The wavelength division multiplexer cannot separate light having two different wavelengths at the rate of 1:1. Thus, the separation of light is imperfect. When the outputs are denoted by 1 and 2, xcex1:xcex2=1:xcex5 is established at the output 1, and xcex1:xcex2=xcex5:1 is established at the output 2. The extinction ratio xcex5 is never zero but is approximately at least 1/100.
Therefore, crosstalk is caused by the sense of unnecessary light caused by reflecting or scattering light, in particular the reflection light of the 1.3 xcexcm light itself, in the connector part and the wavelength division multiplexer. It is a serious fault. Since the wavelength division multiplexer cannot, sufficiently separate the light into two different wavelengths, the dielectric multilayer additionally brings the extinction ratio close to zero. Hence, a filter capable of cutting the 1.3 xcexcm light was required to be inserted between the connector 23 and the wavelength division multiplexer 21 shown in FIG. 5.
A horizontal-type photodiode (waveguide-type), which can sense the light having only one wavelength, has been proposed. In this photodiode, the light having other wavelengths is shut out by the waveguide. This has been developed for detecting only the counter part of light separated by a wavelength division multiplexer. Masahito Shishikura, Yoshihisa Tanaka, Hiroshi Matsuda, Hitoshi Nakamura, Takao Miyazaki and Shinji Tsuji, published an article entitled xe2x80x9cWide tolerance waveguide-type PIN photodiodexe2x80x9d which can be found in the General meeting of Electronics Information Communication Seminars, C-386 p386 (1995).
They describe a photodiode for sensing only 1.3 xcexcm light when the light including the wavelengths of 1.3 xcexcm and 1.55 xcexcm enters a waveguide in parallel with the surface. The paper says that the sensitivity ratio of 1.3 xcexcm to 1.55 xcexcm is 23 dB (200 times) when a proper bias-voltage is supplied. This invention aims at developing a photodiode used for sensing only 1.3 xcexcm light after 1.3 xcexcm and 1.55 xcexcm have been separated by a wavelength division multiplexer. Of course, the wavelength division multiplexer is indispensable for separating light. This photodiode is, however, suitable for taking out only 1.3 xcexcm light, but can not be used for 1.55 xcexcm light which is the object of the present invention.
A conventional photodiode module has three main parts, that is, a wavelength division multiplexer, a filter and a photodiode (light receiving device). The photodiode module combining many costs a lot to produce. This is a fatal demerit for a transmitting and receiving device of bilateral light communication.
It is an object of the present invention to provide a photodiode module having fewer parts than known photodiode modules.
Another object of the present invention is to provide a small-sized photodiode module that costs less to produce than known photodiode modules.
A further object of the present invention is to provide a photodiode module suitable for a long-distance communication.
A still further object of the present invention is to provide a low cost photodiode module that has a low optical loss for practical uses for subscribers.
The Inventors considered why a conventional optical photodiode module are expensive and large.
The ordinary light receiving device used a photodiode capable of sensing light having different wavelengths, that is, a wavelength common-type photodiode. Such a photodiode is likely to be convenient in sensing light with different wavelengths. This matter is, however, a problem. Since the wavelength common-type photodiode has the sensitivity for light having different wavelengths, signal light must have been spatially separated prior to taking out specific light. A wavelength division multiplexer and a dielectric multi-layer were indispensable for the spatial separation. Each intensity of light having different wavelengths is detected by the steps of separating the light having different wavelengths spatially, leading the light to different optical paths and providing a commontype photodiode at each terminal point of the different paths.
The present invention does not use the sensitivity common-type photodiode but uses a photodiode for sensing light having only one wavelength, that is, a sensitivity intrinsic-type photodiode. An intrinsic photodiode Dj is used for sensing only an object wavelength of xcex j. The dielectric multi-layer is not used here. This photodiode has the function of the wavelength division multiplexer. Therefore, any wavelength division multiplexer is not necessary. The invention is provided by deep consideration pertaining to the physics of a semiconductor photodiode. It is feasible for a semiconductor photodiode to sense only a specific wavelength.
The wavelength-dependency of sensitivity of the semiconductor photodiode was explained by FIG. 8. This point is very important for the present invention, so that a more detailed explanation will be provided. The relation between the energy of the incident light and the band structure of semiconductor will be explained by referring to FIG. 22. The energy on the bottom side of a conduction band is denoted by Ec, and the energy at the top of valence band is denoted by Ev. The difference between Ec and Ev is a bandgap, that is, Eg=Ecxe2x88x92Ev. The interval between the conduction band and the valence band is a forbidden band.
When there is no impurity in the semiconductor, the forbidden band has no level. The conduction band has a few electrons but the valence band is condensed with electrons. Holes correspond to a lack of electrons in the valence band. The density of electrons in the conduction band and the density of holes of the valence band are zero at 0K (Kelvin). When there is no impurity level, a Fermi level is positioned at the middle height of the forbidden band. The electrons in the conduction band and the holes in the valence band are a little excited by heat in a definite temperature.
When a photon enters the valence band, a electron in the valence band is excited into the conduction band, which is shown by xe2x80x9caxe2x80x9d in FIG. 22. This phenomenon is called xe2x80x9cexcitation of electron-holexe2x80x9d. This phenomenon takes place only when the energy of light is higher than the bandgap. Hence, an inequality h xcexdxe2x89xa7Eg is satisfied. If a semiconductor has a sufficient thickness, all of the light having higher energy than the bandgap is absorbed. In other words, all light having a wavelength xcex shorter than a wavelength xcexg (=hc/Eg) of the absorption edge is absorbed, that is, xcex less than xcexg.
On the contrary, when the light having a smaller energy than the bandgap enters the valence band, the energy is insufficient, as shown by arrows xe2x80x9cbxe2x80x9d and xe2x80x9ccxe2x80x9d. Since there is no electron level in the forbidden band, the transitions shown by arrows b and c do not occur. Hence, a photon having smaller energy than the bandgap (h xcexd less than Eg) passes through the semiconductor as it is. The semiconductor is transparent to the light having lower energy than Eg. Hence, the light having a wavelength longer than the wavelength xcexg on the absorbing edge can pass through the semiconductor.
The above explanation pertains to the case of intrinsic semiconductors in which there is no electron level in the forbidden band. However, even if the semiconductor is n-type or p-type, shallow impurity levels En and Ep are mostly generated. This will be explained by referring to FIG. 23.
In this case, the limit of energy for causing the transition is (Egxe2x88x92En) or (Egxe2x88x92Ep). These levels En and Ep are from a several hundredth to a tenth of the bandgap. Therefore, the electron level scarcely occurs in the forbidden band, even when a semiconductor is n-type or p-type, but a little deviation of the absorbing edge occurs.
The impurity making a deep level such like Es in FIG. 23 is not doped for controlling the conductivity. The epitaxial layer with a good quality does not have such a deep impurity level. When a semiconductor is n-type or p-type, the limit of photon energy absorbed is (Egxe2x88x92En) or (Egxe2x88x92Ep) instead of the bandgap of Eg. However, there is not a large difference therebetween, the absorption energy is briefly represented by the bandgap Eg.
In any case, photons having a higher energy h xcexd than the bandgap Eg, that is, Exe2x89xa7Eg xcex less than xcexg, are absorbed in the semiconductor but photons having a lower energy than the bandgap (E less than Eg:xcex greater than xcexg) pass through. Hence, the semiconductor has the selectivity of wavelength by itself. Until now, there have been no devices utilizing semiconductors by themselves as wavelength selective devices. The Inventors utilize the wavelength-selectivity of semiconductors, and propose for a first time a device making use of the wavelength-selectivity selectivety of semiconductors. The present invention will be explained by describing in detail various presently preferred embodiments of the invention.
This invention proposes a photodiode having the function of filter by itself. Therefore, this invention provides a module enjoying a cheaper cost and easier handling as a light receiving device for transmitting two sorts of light having different wavelengths due to the filter function. A wavelength division multiplexer is used in the two-wavelength transmission system, but a dielectric filter for complementing the loss of extinction ratio is unnecessary. The above-mentioned is related to two wavelengths of xcex1 (=1.3 xcexcm) and xcex2 (=1.55 xcexcm), that is, (a) is the combination of the 1.3 xcexcm light and 1.55 xcexcm light. This invention can be adopted in any combination of two wavelengths. Other realizable examples of combining two wavelengths are as follows.
b) Combination of 0.98 xcexcm and 1.3 xcexcm
c) Combination of 1.3 xcexcm and 1.65 xcexcm
d) Combination of 1.55 xcexcm and 1.65 xcexcm
This invention is utilized for GaAs-type photodiodes of shorter wavelengths besides InP-type photodiode for long wavelengths.
The filter layer having a band gap wavelength xcexg satisfying xcex1 less than xcexg less than xcex2 absorbs xcex1 and admits xcex2 to pass through. The PD itself has the wavelength selectivity. The filter layer saves the cost of the optical communication by eliminating WDMs. The n-electrode spanning both p- and n- regions cancels the delayed pairs of electrons and holes generated by the stray light at the periphery. The disappearance of the delayed current enhances the tolerance of mounting optical parts and PDs. Since both n- and p- electrodes appear on the top, the PD is suitable for the top surface installment.